1. Field of the Invention
The present invention relates to an extrusion die device and, more particularly, to an extrusion die device for forming a hollow object with an integrally formed helical rib by one-time extrusion.
2. Description of the Related Art
Extrusion includes applying pressure to force a heated metal material to pass through a shaping hole of a die, obtaining a produce having a hollow object or a solid rod. Metal tubes in rehabilitation devices or sport devices have specific requirements in strength. In an example shown in FIG. 1, the metal tube 1 includes a central rib 11 to enhance the bending strength. The metal tube 1 has uniform cross sections, and the central rib 11 provides enhanced strength of the metal tube 1 in the extending direction of the central rib 11. However, the structural strength of the other portions of the metal tube 1 not supported by the central rib 11 may be insufficient. The wall thickness of the metal tube 1 or the central rib 11 can be increased to enhance the structural strength of these portions, but the costs and the weight of the metal tube 1 are both increased.
Formation of helical rods or helical tubes by extrusion is known. Furthermore, helical fins can be formed on an outer periphery of a hollow object by extrusion. However, formation of a hollow object with an integrally formed central helical rib by one-time extrusion utilizing conventional die devices without changing the wall thickness of the hollow object or the central helical rib for providing the hollow object with uniform structural strength in the radial direction is still difficult.